The invention relates to a method and an apparatus for balancing a motor vehicle wheel, in which method the wheel which is mounted on a main shaft of a balancing machine by means of a driving prestressing force is accelerated by means of an AC motor to the measurement rotation speed by a specific drive torque, and is braked by a braking torque after carrying out the measurement run.
In the case of a method and an apparatus of this type which are known from DE 100 00 235 A1, the motor vehicle wheel is accelerated to the measurement rotation speed by the drive torque that is produced by the AC motor. On reaching the measurement rotation speed, a reduced supply current is supplied to the windings of the motor during the measurement run, which essentially compensates for any drop in rotation speed caused by friction and air drag, so that the unbalanced measurement run is carried out at the desired measurement rotation speed. After completion of the measurement run, the motor vehicle wheel is braked, in which case the AC motor can supply the necessary braking torque. In the known apparatus, the AC motor is in the form of a single-phase AC motor.
In order to keep the cycle time which is required for acceleration of the balancing machine to the measurement rotation speed, for the measurement run and for braking as short as possible in order to achieve economical utilization of the balancing machine, the drive motor, in particular a single-phase AC motor, for driving the main shaft is designed to produce an adequate torque, for example in the order of magnitude of approximately 24 Nm. In order to transmit the high torque from the main shaft to the motor vehicle wheel, this motor vehicle wheel must be mounted on the main shaft in a rotationally fixed manner by means of an appropriately high driving prestressing force. Otherwise, there is a risk of the motor vehicle wheel sliding on the main shaft during acceleration to the measurement rotation speed and/or during braking after carrying out the measurement run. However, for accurate compensation for any unbalance, the motor vehicle wheel must remain connected to the main shaft in a rotationally fixed manner and must maintain the same rotation angle position on the main shaft. When balancing wheels with designer rims, in particular composed of light alloy, there is a risk of the wheel disks being damaged by the high clamping forces which are required to mount the motor vehicle wheel on the main shaft. When balancing motor cycle wheels, it is necessary to press the clamping lever(s) (which is/are used to produce the driving prestressing force) onto the tire with a high contact pressure force. This results in a risk of bending of the clamping shaft on which the motor cycle wheel is rotated during the unbalanced measurement run, and hence of displacement of the wheel center point. Exact balancing of the motor cycle wheel is then impossible.
The object of the invention is therefore to provide a method and an apparatus of the type mentioned initially which make it possible to carry out an accurate unbalanced measurement on different wheel types without complex changes to the measurement arrangement.
In the case of the method, this object is achieved according to the invention by the distinguishing features of patent claim 1, and in the case of the apparatus, it is achieved by the distinguishing features of patent claim 3.
In contrast to the conventional operation of balancing machines, in the case of the invention, the torque which is produced by the drive motor during acceleration to the measurement rotation speed is influenced as a function of the type of wheel which is mounted on the main shaft. In the case of motor vehicle wheels whose wheel disks are composed of heavy material, for example steel, the drive motor may produce a high torque, for example of approximately 24 Nm, in order to reach the measurement rotation speed as quickly as possible. A high braking torque may also be produced during braking, in order to bring the motor vehicle wheel to the desired compensation position in the shortest possible time after carrying out the measurement run. In the case of motor vehicle wheels with wheel disks made of light alloy, and in particular with designer rims, the torque which is produced by the drive motor is reduced to 50%, 25% or even less. The torque is also reduced for motor cycle wheels in order that the prestressing force by means of which the clamping lever of the driving apparatus is pressed onto the tire can be reduced sufficiently to avoid bending of the clamping shaft on which the motor cycle wheel revolves during the unbalanced measurement run.
During balancing of a motor vehicle wheel, the torque during acceleration to the measurement rotation speed and the braking torque during braking after carrying out the measurement run can advantageously be adjusted as a function of the type of wheel to be balanced, such that the rotationally fixed connection between the main shaft and the motor vehicle wheel is maintained even when the driving prestressing force between the main shaft and the motor vehicle wheel is reduced.
Reducing the drive torque and braking torque makes it possible to connect motor vehicle wheels with wheel disks made of sensitive material, for example light alloy, and motor cycle wheels to the main shaft with a reduced prestressing or clamping force in a rotationally fixed manner without there being any risk of the motor vehicle wheel sliding, and hence of the measured value being corrupted.